Optical location systems are used to locate a mobile object that either receives or transmits optical signals. Typical systems involve at least one object, i.e., the target, that moves within an area whose perimeter is defined by the range of the transmitter and receiver. In some systems, the target carries the transmitter and the receiver is stationary; in others, the target receives a transmitted signal. In a third type of system, the target reflects transmitted light back to a receiver. An advantage of placing the transmitting device on the target is that the transmitted signal may then contain data identifying the target.
Many optical location systems are analog in the sense that the receiver is slewed into a position close to alignment with an emitting target. At this position, the sensor operates a servo loop to lock the sensor to the target. However, such systems have narrow fields of view and do not accurately follow a rapidly moving target.
U.S. Pat. No. 2,855,539 discloses a digital optical location system in which light is passed through a code plate to a number of point sensors. The code plate contains rows and columns of "windows" of varying sizes, which represent bit positions of a digital code. A lens is used to focus a line image from a target onto one row or column at a time. Whether or not the light passes through a window of that row or column is used to determine a code, and hence the position of a target.
U.S. Pat. No. 3,602,903 discloses a digital optical location system in which a number of sensor units each have two parts. One of the two parts of each sensor unit detects light from the target. The parts are "finger shaped", so as to divide each sensor unit in a binary fashion. As resolution increases, the number of "fingers" for each sensor unit increases. The detection or non detection of light by each part of each sensor unit is used to generate a code representing the position of the target.
A third digital optical location system is described in U.S. Pat. No. 4,710,028 to Grenier. An image frame surrounding the target is focussed onto a matrix of photosensitive elements. The coordinate position of a sensitized element is used to determine the position of the target.
One limitation of existing digital optical location systems is a high correlation between the number of sensors and the resolution of the location matrix. This limitation is significant because of the cost of sensor devices. U.S. Pat. No. 4,710,028 provides a resolution of n sections where n is the number of sensing elements in the matrix. Although U.S. Pat. No. 3,602,903 uses binary coding to provide a resolution of 2.sup.n, where n is the number of sensor units, each sensor unit has two sensors, making the required number of sensors 2.times.2.sup.n.
U.S. Pat. No. 2,855,539 provides a resolution of 2.sup.n, where n is the number of sensors. However, the need to focus line images on rows and columns of a code plate leads to unnecessary complexity and to accuracy problems.
A need exists for an optical location system that minimizes the number and size of sensors required for a desired resolution, and also minimizes accuracy of position finding.